Thank you for your questions.
You can read more about the config.xml file, which is part of Apache Cordova, here
In the context of AppBuilder, here is what you need to know.
- At build time, AppBuilder pulls information from your manually modified config.xml, from config.xml-related project properties such as the enabled plugins, and from any custom plugin plugin.xml files properly nested in your Plugins folder.
- This means that you do not need to manually modify config.xml to contain references to the core or the integrated plugins.
- If the plugin.xml contains the proper <feature> declarations for the custom plugin in the proper <platform> element, you do not need to manually include them in your config.xml. You might want to check the plugin.xml reference here.
- AppBuilder resolves plugin dependencies in a smart manner. This means that even if you have not enabled a core or integrated plugin, required by another plugin you are currently using (be it core, integrated, or custom), AppBuilder will automatically enable it and make the necessary references at build time.